The invention pertains to a process and the device with the features of the preamble to the independent patent claims.
In vat dyeing, textiles are dyed with so-called vat dyes, e.g., indigo, in a special dyeing process. Vat dyes are compounds with indigoid or anthrachinoid structures and are mostly insoluble. For dyeing, the vat dyes are first chemically reduced and transferred into a water-soluble fiber-affinity leuco-dye. The leuco-dye forms the dye liquor, which is also frequently referred to as a vat. The cloth that is to be dyed is generally steeped in the vat. Because of the high fiber affinity of leuco-dyes, the dyeing speed is high and can lead to rregularities, which can be corrected by adding equalizing agents or by raising the temperature. The textiles are then separated from the dye liquor and washed. Because of contact between the liquor-impregnated cloth and atmospheric oxygen or other oxidation agents, the initial dye builds back up through oxidation. Non-water-soluble dye that is produced during this oxidation process adheres very well to the thread, is extremely genuine, and cannot be further modified by atmospheric oxygen.
To produce textiles, warp thread bundles are frequently run through a steep bath filled with a dye liquor. The threads, which enter the liquor at high speed, entrain oxygen from the ambient air into the liquor, thus partially oxidizing the liquor even in the steep bath. This causes a great deal of dye to be consumed which can no longer be used for dyeing, but is simply lost.
Another problem with vat dyeing of threads lies in the fact that the liquor on the surface of the threads, which leave the dyeing liquor at high speed, are immediately oxidized by contact with the oxygen in the atmosphere. The dye is thus fixed on the surfaces of the threads but cannot penetrate into them.
The known way to keep the dye liquor from oxidizing in the steep bath is to supply the liquor with oxidation inhibiting agents. This raises concerns, however, from the ecological viewpoint. Another known way of preventing the dye liquor from oxidizing immediately on the surfaces of threads after leaving the steep bath is to run the threads through an inert-gas atmosphere immediately after leaving the liquor. If the threads remain in a non-oxidizing atmosphere for certain length of time, the liquor can penetrate into the threads sufficiently without being oxidized. Such an arrangement is known from, e.g., FR 1097607.
To prevent oxygen from being entrained by the threads when entering the dye liquor, the threads are also often treated with inert gas before making contact with the liquor. This is known from, e.g., U.S. Pat. No. 5,378,246 and U.S. Pat. No. 5,494,491.
DE 43 42 313 also teaches running the threads, before they enter the dyeing liquor, through the chamber, which is essentially oxygen-free. The purpose of this is to prevent oxygen from being entrained by the threads and incorporated into the dyeing liquor. This device is, however, associated with the disadvantage that the inert atmosphere (an inert gas such as nitrogen, or superheated steam) has to be kept under a certain pressure because of the openings in the housing. Since the housing that surrounds the oxygen-free atmosphere has two openings (entry and exit) for the yarn, the loss of inert gas is relatively large. Inert gas is, however, an important cost factor in the operation of such dyeing systems, and its consumption should therefore be kept as low as possible. Another disadvantage of such devices lies in the fact that, despite all efforts to the contrary, oxygen is able to penetrate into the essentially oxygen-free chamber through the entry lock in the form of a crushing mill. This has the effect that the holding section for the dyeing liquor to be absorbed into the yarn cannot be kept sufficiently oxygen-free, or can be kept so only at high cost.